Reports on bombings in Syria these days have become routine. But when Mohammad Sabbagh, an industrialist from Aleppo, heard about the attack near his hometown on March 19, the details stopped him cold. Survivors and witnesses of what was being described by the government news agency as a chemical attack said they smelled something like chlorine. And as the owner of Syria’s only chlorine-gas manufacturing plant, Sabbagh knew that if chlorine was involved, it most likely came from his factory.

The attack killed 31 people, including 10 soldiers, and wounded scores more. In the immediate aftermath, the Syrian government and the opposition traded accusations. The government claimed that “terrorists,” its term for the rebels that have been fighting the regime for two years, had fired a “missile containing a chemical substance” at the village of Khan al-Asal in retaliation for their support of the government. Kasem Saad Eddine, spokesperson for the opposition military council of Aleppo, accused the government of attacking its own people in order to smear the opposition. “The regime is trying to hide its crime by accusing the FSA,” he tells TIME, referring to the Free Syrian Army, the loose confederation of rebel groups fighting the government. Eddine also accused the Syrian government of launching a second chemical attack near Damascus, causing an unspecified number of casualties. Whatever the case, the attack at Khan al-Asal marks a chilling evolution in a war that has already taken 70,000 lives and disrupted, perhaps permanently, millions more. If it turns out that the government has used chemical weapons, international demands for armed intervention will increase. If the rebels used them, the escalation in tactics indicates that the war is about to become even bloodier.

The U.N. has acquiesced to a Syrian government request to send an investigation team to Khan al-Asal; it is expected to arrive on site this week. The team will be headed by Ake Sellstrom, a veteran chemical-weapons inspector from Sweden who was instrumental in investigating and dismantling Iraq’s chemical- and biological-weapons programs in the 1990s. It is not yet clear if the U.N. team will investigate other accusations of chemical-weapons use in Syria, nor is it clear how much access it will have. The final details for the trip will be worked out in the coming days. The team’s mandate is limited to a technical investigation, which means it will only be able to ascertain whether or not chemical weapons were used, not who used them — a frustrating outcome for those seeking clarity. Nevertheless, the findings could be a strong indication of who might have been behind the attacks. The Syrian government is believed to possess one of the biggest stockpiles of chemical weapons in the world, including nerve agents Sarin and VX, as well as mustard gas, though it has repeatedly said it would never use such weapons against its own people. The opposition, though it also says it would never use chemical weapons, does have access to at least one item that could be used in a chemical attack: Sabbagh’s chlorine gas.

In August rebel forces took Sabbagh’s factory by force, as part of a sweep that also netted them an electricity station and a military airport about 30 km from Aleppo. Sabbagh, who has since fled Aleppo for Beirut, says his factory is now occupied by Jabhat al-Nusra, a militant group with strong ties to al-Qaeda that has been designated a terrorist group by the U.S. He knows this because his site manager has struck a deal with the rebels — they supply 200 L of fuel a day to keep the generator running so that the valves of his $25 million factory don’t freeze up. The factory isn’t operational anymore, but this way at least, says Sabbagh, it might be one day in the future. In the meantime, he has no idea what has happened, if anything, to the 400 or so steel barrels of chlorine gas he had stored in the compound. The yellow tanks, which hold one ton of gas each, are used for purifying municipal water supplies. “No one can know for certain, but if it turns out chlorine gas was used in the attack, then the first possibility is that it was mine. There is no other factory in Syria that can make this gas, and now it is under opposition control,” he says.

To Faris al-Shehabi, head of the Aleppo Chamber of Industry and a strong government supporter, it was obvious from Day One that the rebels had their eyes on the gas. “Why else would they capture a factory in the middle of nowhere? For the sniper positions?” he asks sarcastically while meeting TIME in Beirut, where he is traveling for business. “We warned back then that chemical components were in the hands of terrorists, but no one listened.”

The investigation, when it starts, will be hobbled by the passage of time. According to a chemical-weapons expert familiar with such inquiries, who spoke on condition of anonymity over the telephone, the investigating team will examine soil, air and oil samples taken from the blast site. It is unclear whether the team will have access to survivors (who probably bear little traces of the chemicals so long after the attack) or to autopsy reports. But initial assessments based upon body counts, photos and video footage taken at the hospital after the attack seem to rule out nerve agents or mustard gas. “Looking at the death rate relative to the number of people exposed, it couldn’t have been a weaponized nerve agent,” says the expert. “And mustard gas rubs off on whoever touches it, but you don’t see the medical personnel taking additional protective measure when they treat the patients. So it’s pretty likely it was something else.”

A doctor who treated victims immediately after the attack, and who asked not to be identified, said few of the patients had visible wounds. Most suffered from severe cramps, vomiting, headaches and troubled breathing. Those who died did so right after breathing the gas, he says. “Our staff are not used to dealing with such cases, it was the first time we treated something like it.” Several hours after the attack, two doctors returned to the blast site to warn people away. The smell, described as “rotting garbage,” was still there.

As unlikely a deliberate use by the Syrian government of a nerve gas might be, especially considering the near certainty of an international reaction, few believe that the opposition has the wherewithal to make even crude chemical weapons. “It’s a question of capabilities,” says Greg Thielmann, a chemical-weapons expert at the Washington-based Arms Control Association. “Even if they had chemicals, where would they get access to a delivery system?” More likely, he says, the mass poisonings were a “side effect of a high explosive device that released a chemical in the vicinity.”

Who might have detonated that kind of improvised chemical explosive device, however, is impossible to tell. Government supporters frequently make the link between Jabhat al-Nusra and the al-Qaeda-affiliated militants in Iraq who experimented with chlorine bombs at the height of that country’s sectarian war in 2006 and ’07. There, insurgents rolled the yellow barrels of chlorine gas onto the back of pickup trucks packed with explosives. The effects were negligible, says the chemical-weapons expert, who points out that the heat of the blast tends to evaporate the chlorine, diminishing impact. Still, by linking al-Qaeda’s failed chemical-weapons attempts in Iraq with allegations that Jabhat al-Nusra is doing the same, the Syrian regime can buttress its long-standing charge that the rebellion against its rule is being driven by radical Islamists.

Rebel spokesman Eddine says the allegations are ridiculous. He holds that the regime was actually trying to target FSA rebels who had taken a nearby military school, and missed, hitting the progovernment village nearby. To progovernment industrialist al-Shehabi, the idea that the Syrian army would use crude chemical weapons doesn’t make any sense. “Why would the army use primitive chemical weapons against civilians in a not-so-important area, and not use the real chemical weapons we supposedly have when we are faced with our enemies taking over our strategic military bases?”

Both believe that the pending U.N. investigation will clear everything up. In fact, the investigation, says weapons expert Thielmann, is likely to make the situation more complicated — especially if it turns out, as suspected, that a substance just short of a proscribed chemical agent was used. Citing U.S. President Barack Obama’s warning nearly a year ago that the use of chemical weapons by the Syrian government would be a “red line” for U.S. intervention, Thielmann points out that chlorine might not qualify. “It becomes a very fuzzy red line. Was Obama only talking about chemical weapons prohibited under the Chemical Weapons Convention? Well, depending on how you look at it, chlorine would not be prohibited.” And Syria’s brutal civil war grinds on.

Aryn Baker is the Middle East Bureau Chief for TIME, covering politics, society, culture, religion, the arts and the military in the greater Middle East, including Pakistan and Afghanistan. She currently resides in Beirut, Lebanon.